Dissociative Attachment of Electrons in Iodine. II. Mass Spectrographic Determination of the Energy Dependence of the Cross Section

The energy dependence for electron attachment in iodine vapor is studied in a mass spectrometer and a total ionization tube using a collimated electron beam. The dissociative attachment process ${\mathrm{I}}_2+e\to {\mathrm{I}}^{-}+\mathrm{I}$ is found to be the only significant one. In the mass spectrometer the electron energy is determined by retarding potentials applied to the electron beam. In the total ionization tube a study of S${\mathrm{F}}_6^{-}$ is used to establish the electron energy scale and to determine the electron energy distribution. These studies show that the maximum attachment cross section occurs for essentially zero-energy electrons, and the measured cross section decreases to half-value in about 0.03 ev. The mass spectrometer studies show the disturbing presence of ${\mathrm{I}}^{-}$ from hydrogen iodide which is presumed to be formed from the reaction of dissociated iodine with the water of crystallization in the glass.